1. Field of the Invention
The present invention relates to improvements in dynamoelectric machines having an electro-insulative, heatdissipating housing in which coil windings and a core of a stator are tightly and voidlessly molded, and more particularly, to dynamoelectric machines having improved electric and mechanical properties that are manufactured by the use of a resin-filler system with molding characteristics optimized for stators of dynamoelectric machines.
2. DESCRIPTION OF THE PRIOR ART
U.S. Pat. No. 3,758,799, titled "DYNAMOELECTRIC MACHINE", to Dochterman et al, discloses dynamoelectric machines having a housing which encapsulates stator coil windings and a core with a resin-filler composition system. The housing disclosed in the patent does not include a metal case or shell and does include a substantially rigid structure member of an interstitial mass of inert particulate material.
Adhesive material such as an unsaturated polyester resin occupies interstices of the mass and bonds inert particulates together and to remainder of the structure.
This patent discloses several kinds of resin-filler composition systems, which are most important for the dynamoelectric machines of this type. From the practical point of view, the dynamoelectric machines are manufactured by injection or transfer molding; therefore, characteristics of the resin-filler composition system must be fully investigated and optimized, otherwise the dynamoelectric machines of this type cannot be produced on an industrial basis. For example, in small sized motors or generators, coil windings made of fine wires are easily deformed during molding operation of the housing. Furthermore, in the motors or generators, slots formed in the inner periphery of a stator core are very narrow in width. If the resin-filler composition system has unsatisfactory flow properties, filling of the slots is insufficient. If the resin-filler composition system has a viscosity which is too small, resin separation will be large so that electrical properties of the stators of the dynamoelectric machines will be unsatisfactory.
There are still many other factors to be considered when stators of dynamoelectric machines disclosed in the U.S. Patent are manufactured by injection or transfer molding.
There have been proposed a variety of resin-filler composition systems such as disclosed in U.S. Pat. No. 3,658,750, titled "THERMOSETTING RESIN COMPOSITION AND ELECTRICAL APPLIANCES USING THE SAME" to Tsukui et al, and U.S. Pat. No. 3,763,080, titled "PRESSURE MOLDED ARTICLES AND MOLDABLE COMPOSITION", to Deuter. Tsukui et al disclose a resin-filler composition usable for transfer molding of rotor coils of motors, and of transformers, which comprises a thermosetting resin, a coarse powder filler having a particle size larger than 100 .mu.m and a fine powder filler having a particle size smaller than 60 .mu.m, an amount of the fillers being 40 to 80 percent by volume of the total volume. Tsukui et al also disclose addition of finely divided lead tetraoxide powder as a precipitation inhibitor. Investigations by the present inventors have shown that the resin-filler compositions disclosed in the Tsukui et al patent are not useful as a molding material for stators of small-sized dynamoelectric machines. The cured product of the compositions exhibited a small crack resistance and unsatisfactory flow properties when applied to stators of dynamoelectric machines.
Deuter discloses a moldable composition comprising a thermosetting resin, a particulate material having a particle size distribution such that about 95% to 100% passes a 30 mesh sieve, at least 50% passes a 40 mesh sieve and is retained on a 100 mesh sieve, and no more than a minor amount passes a 200 mesh sieve and a separation inhibitor. Deuter exemplifies as a separation inhibitor colloidal silica, aerosol Grade 200, powdered silica, powdered asbestos, extremely short glass fibers and powdered mineral wollastonite. Further, there are exemplified as a particulate material chromium ore, slate, chalk, zirconia, alumina calcium, carbonate, mica, beryllium oxide, magnesium oxide and sand. The particulate material should have a much larger particle size than the separation inhibitor. In other words, the particle size of the former is 40 to 100 mesh (more than 149 .mu.m in diameter), while the particle size of the latter is about 0.012 .mu.m in case of colloidal silica (Cab-O-Sil M-5).
According to the description of the Deuter patent, suitable separation inhibitors should have a particle size of 325 mesh or less (44 .mu.m or less in terms of particle size). Investigations of the moldable composition disclosed in this patent showed that its molding characteristics were unsatisfactory and electric properties of the resulting housing of the stator were not good.
U.S. Pat. No. 3,562,201, titled "UNSATURATED POLYESTER RESIN/MONOMER MOULDING COMPOSITIONS", to Crowe et al., discloses a composition which comprises an unsaturated polyester resin, a primary absorbent filler and a secondary filler. The primary absorbent filler includes diatomite, bentonite, etc. and is able to absorb liquid from the polyester/monomer system to thereby improve flow properties of the composition. The secondary fillers, though not essential for the composition, include such as glass fiber, minerals, e.g. finely divided calcium carbonate, clay, alumina trihydrate, dolomite, asbestos, or synthetic fibers. Crowe et al. do not describe the particle size of the primary absorbent filler, but the present inventors believe the particle size thereof is very small because "Dicalite" used as the primary absorbent filler has an extremely small particle size. However, this composition has been found to be improper when applied to the injection or transfer molding of stators of dynamoelectric machines, especially for small sized dynamoelectric machines, because it showed unsatisfactory molding characteristics including flow properties.